Aircraft control surfaces, for example flaps located on the trailing edge of a fixed wing, slats located on a leading edge of a fixed wing, spoiler panels, aileron surfaces, and the like, have traditionally been actuated by hydraulic actuation systems. More recently, electromechanical actuators (“EMAs”) have gained acceptance in the aviation industry for adjusting the position of control surfaces. Known EMAs have a motor-driven ballscrew shaft mated with a ballnut. The ballnut is engaged by a surrounding spline member which prevents rotation of the ballnut while permitting axial movement of the ballnut. Thus, when the ballscrew shaft is rotated, the ballnut moves axially along the ballscrew shaft to produce linear drive. In some configurations, a brake is associated with the motor drive shaft to stop rotation and hold the ballnut at a commanded stroke position of the EMA.
The prior art has some shortcomings. For example, there is only one load path through which the actuator stroke is produced. In the event of a motor failure or mechanical jam preventing or interfering with rotation of the ballscrew shaft, there is no redundant load path through which the EMA may be operated. In configurations where a brake is used to hold the EMA at a commanded stroke position, the brake requires its own control circuitry and must be actively commanded, adding complexity to the control system architecture. In some prior art systems, motor power is maintained even when the EMA is in a braked state. In other prior art systems, a motor “power off” command is used when the EMA is in a braked state.
What is needed is an improved EMA offering redundant load paths that may be passively locked to hold the EMA at a commanded stroke position.